Hizbullah has sent security officials to Damascus to help investigate the assassination of Imad Mughniyeh, the organisation's military commander, it emerged today as Israel ordered an alert in case of retaliatory attacks.

Arab media reported that Hizbullah was cooperating with the Syrian and Iranian authorities to trace the killers of Mughniyeh - hailed as a "martyr" of the resistance by the Lebanese Shia group and condemned as a terrorist by Israel and the US.

Tuesday's deadly car bombing was deeply embarrassing for Syria, both as a breach of its normally tight security and as exposure of the hospitality it grants to militant groups. The Palestinian movements Hamas and Islamic Jihad also have offices in Damascus.

Lebanese sources said last night that several suspects, mostly Palestinians residing in Syria, had been arrested.

Israel's chief of staff, meanwhile, ordered the armed forces to raise their alert level on the ground and in the air in order to protect the country's northern border from Hizbullah. Security is also being intensified at Israeli embassies and consulates abroad.

The announcement was a sign that Israel is taking Hizbullah seriously after its leader, Sheikh Hassan Nasrallah, pledged at Mughniyeh's funeral in Beirut yesterday that there would now be "open war" with the Jewish state.

Israel has denied being behind the killing but experts say it has the hallmarks of a Mossad operation. Israel has a track record of liquidating dangerous enemies, including a previous Hizbullah leader, Abbas Musawi.

That was followed in 1992 by the car bombing of Israel's embassy in Argentina. Two years later Israel's capture of a top Hizbullah commander brought the bombing of a Jewish cultural centre in Buenos Aires.

In 2006, Nasrallah vowed to act to free Lebanese prisoners in Israel, and then staged a daring cross-border raid to kidnap two Israeli soldiers to use as bargaining chips. Hizbullah then fired thousands of rockets in the resulting month-long war in which 1,200 Lebanese were killed.

Diplomats said that one consequence of the Mughniyeh killing would be an end to any hopes of a deal involving the two Israeli soldiers or their remains, if, as is widely believed, they are dead. Germany and the International Red Cross had been trying to broker a swap.

"Nasrallah's rhetoric at the funeral ceremony was very harsh," said one senior western official. "They will be cautious about attacking across the border for fear of the consequences in Lebanon. But the fear is that Hizbullah will now start bombings and kidnappings again.

"If they do it's the end of the road. There will certainly be revenge for what happened. Whether it will be a one-off or a military target are questions to ask. They will be careful, but they'll do something"

Nasrallah infuriated Israel last month when he boasted that his organisation had the body parts of several soldiers - triggering angry calls for his assassination.

In Baghdad today the radical Iraqi Shia cleric, Muqtada al-Sadr, paid tribute to the Hizbullah leader: "The dirty hands have stretched out to get [Mughniyeh] and he was sent to paradise where he became a martyr of world Islamic resistance."